This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        DL downlink (eNB towards UE)        eNB E-UTRAN Node B (evolved Node B)        E-UTRAN evolved UTRAN (LTE)        FDD frequency division duplex        HO handover        LTE long term evolution        MAC medium access control        MM/MME mobility management/mobility management entity        Node B base station        O&M operations and maintenance        OFDMA orthogonal frequency division multiple access        PDCP packet data convergence protocol        PHY physical        PRACH physical random access channels        RA random access        RACH random access channels        RLC radio link control        RRC radio resource control        RSRP reference signal received power        SC-FDMA single carrier, frequency division multiple access        S-GW serving gateway        TDD time division duplex        TTI transmission time interval        UE user equipment        UL uplink (UE towards eNB)        UTRAN universal terrestrial radio access network        
A communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. As presently specified the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.6.0 (2008 September), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8).
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (SGW) by means of a S1 interface. The S1 interface supports a many to many relationship between MMEs/Serving Gateways and eNBs.
The eNB may host the following functions:                functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
As illustrated in 3GPP TS 36.300 V8.4.0 (2008 March), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8), a random access (RA) procedure may be performed for at least the following five events:                1) Initial access from a RRC_IDLE state;        2) Initial access after a radio link failure;        3) A handover (HO) requiring a random access procedure;        4) DL data arrival during a RRC_CONNECTED state requiring a random access procedure (e.g., when the UL synchronization status is “non-synchronized”); and        5) UL data arrival during a RRC_CONNECTED state requiring a random access procedure (e.g., when UL synchronization status is “non-synchronized” or there are no PUCCH resources for SR available.        
The random access procedure in 3GPP TS 36.300 may take a contention based form, which is applicable to all five events, and a non-contention based form, which is applicable to the handover and DL data arrival events. The random access process may include four steps for a contention based RA or three steps for a non-contention based RA, the steps corresponding to a message either from the UE to an eNB or from the eNB to the UE. These messages include: a RA preamble from the UE to an eNB (e.g., message 1); and a RA response from the eNB to the UE (e.g., message 2).
Once the RA preamble is transmitted, the UE 10 may monitor a PDCCH in the following TTI window for a random access response. The UE 10 could stop monitoring after a successful reception of a random access response corresponding to the RA preamble transmission.
A random access response window (RAR window) is defined as a subframe window where the UE 10 monitors the PDCCH for the possible random access response following a random access preamble transmission. As described in 3GPP TS 36.300, an example length of an RAR window may be between 2 ms to 10 ms, with an offset of 2 ms.
Multiple available PRACH resources as well as one dedicated preamble may be assigned via message 0 (e.g., a RA preamble assignment), to an UE 10, e.g., a UE 10 which has a DL data arrival or handover requirement. A dedicated preamble may be used by the assigned UE and not by any other UEs within a certain time (e.g., an expiry time). Multiple available PRACH resources are a subset of all PRACH resources in the system and may be spread along the time domain (in FDD and TDD) or the frequency domain (in TDD) as shown in 3GPP TS 36.211 v8.2.0 (2008 March), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8).
In TDD systems, if a preamble format 4 is used, 1 to 6 PRACH resources may be located along the frequency domain in the special subframe. The concepts of a PRACH Mask Index and validity pattern are used to indicate these available PRACH resources.
For further reference see R2-085260, Dedicated Preamble Assignment, Ericsson, RAN2#63b, Prague; R2-085261, Text Proposal for Dedicated Preamble Assignment, Ericsson, RAN2#63b, Prague; R2-085816, PRACH resource indication for dedicated preamble, CATT, Huawei, ZTE, RAN2#63b, Prague; and R2-085082, PRACH assignment for dedicated preambles, Nokia Corporation, Nokia Siemens Networks, RAN2#63b, Prague.
As shown in 3GPP TS 36.321 v8.2.0, when more than one PRACH resource is available in the same subframe (TDD), one of the PRACH resources are randomly selected for use. Any one of the allowed PRACH resources may be chosen with equal probability. This regulation corresponds to a contention based random access.